A read channel integrated circuit (IC) is one of the core electronic components in a modern magnetic recording system, such as a hard disk drive. A read channel converts and encodes data to enable the heads to write data to the disk drive and then read back the data accurately. The disks in a drive typically have many tracks on them. Each track typically consists of mostly user or “read” data sectors, as well as control or “servo” data sectors embedded between the read sectors. The servo sectors help to position the magnetic recording head on a track so that the information stored in the read sectors is retrieved properly.
The servo data format includes a servo address mark (SAM) that is typically the same pattern of bits for all servo sectors. In a Disk Locked Clock (DLC) system, the distance between the servo address marks in consecutive servo wedges, often referred to as the “SAM2SAM distance,” is used to estimate and correct the frequency error of the sampling clock. The SAM2SAM distance is typically obtained using a counter that measures the number of time intervals between consecutive SAM patterns (and has a resolution that is equal to the sampling period, T). The precision of the SAM2SAM distance value is typically improved by concatenating the integer counter value with a fractional fine phase estimate that is based on phase information from the Phase Locked Loop (PLL) in the read channel (and has a sub-T resolution). This fine phase estimate, however, is prone to errors in the presence of large frequency offsets.
Thus, a need exists for improved techniques for measuring the SAM2SAM distance in magnetic recording systems, such as hard disk drives.